1. Field of the Invention
The present invention generally relates to forming solder connections and, more particularly, to the formation of solder preforms on electronic circuit packages at small size and close spacing.
2. Description of the Prior Art
Soldering has been a familiar technique for forming electrical and/or mechanical connections between metal surfaces and is the technique of choice for many applications in the electronics industry. Many soldering techniques have therefore been developed for applying solder to surfaces or interfaces between metals to extend soldering techniques to many diverse applications.
In the electronics industry, in particular, the trend toward smaller sizes of components and higher integration densities of integrated circuits has necessitated techniques for application of solder to extremely small areas and in carefully controlled volumes to avoid solder bridging between conductors. One widely used technique, particularly for direct chip attachment to a board or module, sometimes referred to as "flip-chip" (or surface mount technology (SMT) as to the generic geometry of components), is the use of preforms such as so-called C4 preforms, possibly including a desired flux, for application to locations where solder connections are to be made. Such preforms have been highly successful for forming solder connections at small sizes and close spacings since the volume of solder contained in each preform is accurately controllable. However, the minimum size of such preforms is limited to sizes which can be efficiently handled by automated equipment and does not answer current needs for many electronic package manufacturing applications.
Another widely used technique is to provide pulverized solder material in a viscous binder in the form of a paste which can be applied by stenciling techniques. While this technique has been largely successful in providing application of solder to locations having sizes and spacings smaller than those where solder preforms can be used, the accuracy with which the dispensed volume of solder can be controlled is limited by the stenciling process due to the possibility of contamination, wear and damage to the stenciling masks through which the paste is extruded. Irregularities of solder paste distribution are also caused by the separation of the mask from the surface onto which the solder paste is stenciled. Also, the minimum size of particle of solder material which can be formed is limited by the process by which the particles are formed. That is, particles of smallest size are formed by atomization and solidification of liquid solder causing increase of the ratio of surface area to volume as size decreases, causing increase of oxide to metal volume for a fixed thickness of oxide on the surface of the particle and greater viscosity of the paste for a given metal loading of the paste with the particles. Lowering metal loading and viscosity, in turn, requires a thicker stencil to obtain the desired volume of metal with higher aspect ratio openings, contrary to the requirements of a stencil to accommodate small feature sizes since high aspect ratio stencil openings (and high viscosity) reduce the ability of the paste to release from the stencil. Thus, there is a trade-off between process complexities and requirements which limits the deposit size and stencilling resolution which can be achieved and control of the locations to which either the paste or the solder, itself, may flow.
Further, stenciling processes and the processes for fabrication of masks through which stenciling is done does not support the close spacing or fine pitch of solder connection locations which can be formed by lithographic technologies. Registration of the mask with connection locations also becomes difficult when close spacing of connections is required.
It is also known that, in the process of making a solder connection to a copper conductor, some copper is removed from the copper and becomes part of the solder connection material. This may become critical in some applications in microelectronic manufacturing, particularly in devices which are subjected to high temperature operation and thermal cycling, since tin-copper intermetallic compound precipitates may be formed. Further, the solubility of copper in typical solder materials is very small and on the order of 0.3%. Therefore, most excess copper in the solder materials will be in the form of such intermetallic compounds.
Small amounts of copper in the solder material also degrade the reflow characteristics of the solder. Specifically, when conductors are closely spaced, it is desirable that the solder tend to pull back toward the conductor on which the connection is made and away from adjacent conductors. This action also maximizes the conductive material in the connection and provides for a stable configuration of the solder material even when softened by normal or abnormal temperatures after the device is put into service. Such reflow characteristics may be entirely destroyed by very small copper content in the solder material.
Such reflow may also be adversely affected by small amounts of copper on the surrounding substrate, allowing such areas to be partially wetted or bridged. The only solution at the present time is to dissolve such copper deposits in the solder (which is difficult due to the low solubility of copper in solder materials, especially when the conductor also provides a source of copper solute in the solder material), react the copper from the solder with other materials or use aggressive fluxes. Any of these solutions require long reflow times and may compromise the integrity of the solder connection formed. Other materials, such as gold, are also known to have low solubility in solder and exhibit similar adverse effects on solder connections and reflow.